Innes Sim, a researcher for the RSPB, has just finished a three-year research project studying buzzards along the Welsh border. Although his census of soaring adult birds is not precise, it gives a good guide to the number of breeding pairs, and the results have surprised even the experts. A preliminary analysis suggests that the population density has roughly doubled since 1983.
One reason for the raised eyebrows is that, despite the increase in numbers, so far buzzards have not expanded their range to the same extent. Radio tracking of birds in Dorset may answer that conundrum. It seems that buzzards are conservative: "Although the young birds disperse widely at first, they tend to return to their nest area when they reach breeding age," explains Chris Mead, of the British Trust for Ornithology.
As a result, until recently the rate of expansion eastwards has been limited to around 1 kilometre a year, but there are now signs that densities have reached saturation: "There is a point at which a shortage of food and nest sites will force them to be more adventurous in looking for new breeding territories," says Innes Sim.
Buzzards soaring on their broad, 4ft wingspans are now common sights above some motorways, and in just a few years they have spread east from Bristol as far as Newbury.
The expansion is not confined to major roads. Scotland and Ireland have seen big increases and even the relatively buzzard-unfriendly geography of Suffolk has just scored its first success in a century.
Not surprisingly, however, the most spectacular expansions are along the frontiers of their Welsh, Scottish and West Country strongholds. Wiltshire and Avon have seen dramatic increases, with the number of sightings reported to the Oxford Ornithological Society doubling between 1993 and 1994 alone, while breeding pairs in the area have rocketed from three in the mid-Eighties to about 20 today.
So what underlies this rapid growth in numbers? "It is probably linked to the explosion in rabbit numbers," says Innes Sim.
The result has been a succession of record-breaking breeding seasons: "Using BTO data, the RSPB has found that for the last 10 years or so buzzards have produced more young than at any time since the Fifties," says Sim. Though an average of 1.7 fledglings surviving from each successful breeding attempt may seem low, in fact it represents rapid population growth. Buzzards are long-lived and for the population to remain static, each pair has to produce only two young which reach adulthood, from perhaps 10 breeding attempts.
This is a rare environmental story - representing unqualified good news. Unlike the voracious goshawk, these large raptors prey almost exclusively on pests: "They're not fussy," says Sim. "During the breeding season they take a lot of rabbits, but we've also found they've eaten rats, moles and young crows."
This is not to say they escape persecution. The RSPB says more buzzards are killed illegally than any other bird of prey. As inveterate scroungers, they are prone to poisoning from illegal baits left for crows and foxes; their slow flight makes them tempting for the trigger-happy; and their noisy breeding calls mean their nests are easy to find. Nevertheless, Sim says, most keepers now recognise that buzzards are harmless.
Chris Mead, however, believes that persecution explains much of the buzzard's slowness to spread east. His theory is that deaths within an existing range cause young birds to stay and fill the vacuum, rather than disperse to uncolonised areas.
But, he says, the magnificent sight of buzzards soaring effortlessly on thermals is likely to become more common. And, though they will never be plentiful in flat, open areas of the South-east, "sooner or later they'll certainly get there."